Photolithography is an important step in the manufacture of many highly miniaturized devices. Semiconductor devices, thin film recording heads and micro-machined devices are examples of those made using photolithography. Semiconductor device circuits are being miniaturized more and more. Increased miniaturization has made precision in photolithography processes increasingly more critical. Photolithography tools are machines used to perform photolithography processes in the production of semiconductor devices, thin film recording heads, and micromachined devices.
Semiconductor devices that are used for integrated circuits (ICs) can be formed of twenty or more layers. Typically, each layer requires a unique photomask. After an IC design has been completed, it must be verified and then the process proceeds into the photomask creation step. Photomasks are a very important element in the lithographic process of semiconductor manufacturing. Generally, a reticle, photomask, or mask, comprises a high quality quartz or glass plate containing precision images of different layers of an integrated circuit. The mask is used to optically transfer these images onto semiconductor wafers.
In many cases, after the design is verified, it is converted into GDSII format files, which the mask making process converts into design data that describes the pattern to a mask generator. A layer of photosensitive resist is applied over a chrome layer on top of a blank mask, and the mask generator uses an electron beam or a laser to write the pattern onto a layer of photosensitive resist to produce a pattern in the photosensitive resist layer.
The patterned layer of photosensitive resist is then developed. This exposes the underlying chrome only where the circuit pattern is desired. The bared chrome is then etched through. After etching, the remaining resist is completely stripped away, leaving the circuit image as transparent patterns in the otherwise opaque chrome film.
The photomask, “reticle” or “mask,” contains the patterns used for photolithographic manufacture of integrated circuits. This process is complicated and requires extreme stability in the imaging system and reticle/mask to ensure precision imaging. For example, a photolithographic tool can use the photomask to print circuit elements smaller than 0.15 microns and align them with a precision of a few nanometers. The circuit elements (transistors) are produced on large silicon wafers. Typically, the IC manufacturing process involves transferring a circuit pattern into a resist film that has been coated onto the semiconductor wafer.
The IC manufacturing process generally requires the manufacture of a photomask for each layer of the circuit. The manufacturing procedures and equipment used for photomask generation require the best precision and reproducible imaging technology known. The quality of the reticle can impact the yields of the actual IC manufacturing.
The photomask is subjected to local temperature variations during photomask manufacturing. These temperatures result in temperature gradients and thermal stresses that can adversely affect mask making precision. Under the circumstances, it has been strongly desired to control the heat treatment temperature of the photomask more accurately during its manufacture. The control of the temperature of a mask during its manufacture is complicated by the difficulty in monitoring the local temperatures throughout the mask on a real time basis. Such temperatures and their profiles change on continuing basis during the manufacturing processes. Similarly, mask temperature variations in processes in which such masks are used to make semiconductor devices or other devices can adversely affect the precision of the photolithographic process and degrade the quality of the devices being made.
In a photolithography process of the type used for manufacturing a semiconductor device or a liquid crystal device (LCD), for example, resist is coated on a substrate, and the resultant resist coating film is exposed to light and developed. Such a series of processing can be carried out in a coating/developing system. The coating/developing system has heating sections such as a prebaking unit and a postbaking unit. Each of these heating sections has a heating apparatus with a built-in heater of a resistance heating type.
A key requirement for the processing of wafers is the accuracy of the reticle/mask used to control the features on the wafer. Many steps in the manufacturing of the reticle/mask can cause inaccurate reticle/masks to be produced. For example, variations in critical dimensions (CDs) of the reticle/mask can be caused by variations in thermal profile across the reticle/mask during thermal processing steps and variations in thermal response can also cause matching problems to occur between different reticle/masks and reticle/masks produced at different times.
The manufacturing process for a reticle/mask also requires the application of a resist and the resulting heating steps, exposure step, and development step. For example, prebaking and postbaking are performed under heat treatment conditions according to individual recipes having predefined limits. When the reticle/mask temperature is outside the acceptable temperature range, an acceptable reticle/mask cannot be manufactured.